The present invention relates to a substrate for a printed circuit for use in computers or various kinds of communication appliances.
With the advance of computers or various kinds of communication appliances, demand for printed circuit substrates of glass fiber-epoxy resin has increased and characteristics required for the printed circuit substrates have become higher and higher as the communication appliances have been made to have high performance and have been miniaturized.
In the form of glass fiber used in printed circuit substrates, there are various kinds of materials, such as woven fabric or cloth, nonwoven fabric (including paper type nonwoven fabric), choped strand mat, continuous strand or swirl mat or the like. These materials are used appropriately in combination in accordance with the purpose of use, but the most general material of them is woven fabric or cloth. Generally, as shown in FIG. 3, six to eight sheets of woven fabrics 131 are laminated and impregnated with synthetic resin, and then metal foil 135 is mounted onto at least one of the outermost surfaces of the laminate. The thus laminated body is heated and pressed to form a substrate.
A substrate using glass fiber fabric has a great advantage of excellent reinforcement effect, since a glass fiber content can be increased and a mechanical strength of the glass fiber fabric is large. In the case where the glass fiber fabrics are laminated to make a substrate for a printed circuit, however, there is a problem in surface smoothness and there often occurs a defect of delamination, due to inherent factors of fabric. That is, a glass fiber forming fabric is generally a mono-filament having a filament diameter of from 5 to 9 .mu.m. Several hundreds of filaments are bound and slightly twisted for the purpose of keeping the form of a thread, to use as a warp or weft. Accordingly, the glass fiber filaments forming fabric have a degree of freedom greatly restricted by twisting and weaving processes. In the conventional substrate for a printed circuit, as shown in FIG. 4, the glass fiber fabrics 131 are unified by the synthetic resin 132. The warps 133 and wefts 134 forming each fabric 131 are made compact and tightened, and there is a gap or clearance between the adjacent warps 133 and 133. In this case, the synthetic resin 132 exists mainly between the adjacent fabrics 131, so that the glass fiber fabrics 131 are made into the laminated and stuck state through the resin. Additionally, the metal foil 135 mounted on the surface of the laminate is made wavy microscopically by the intersection between the warp 133 and weft 134 of the upper most glass fiber fabric 131, so that the surface smoothness thereof is unsatisfactory.
Such a problem in surface smoothness has become a large factor for the improvement in wiring density in a printed circuit with the advance of high performance and miniaturization of appliances and a large factor for the reliability of circuits with the advance of fineness of patterns. This is the problem to be solved in the art of substrates for printed circuits. Although the surface smoothness of the standard double-face board (1.6 mm thick) is about 7-9 .mu.m, an improvement to make the surface smoothness to about 3 .mu.m is required today.
Since each of the warp 133 ad weft 134 forming the glass fiber fabric 131, as shown in FIG. 4, is formed by binding and twisting several hundreds of glass fiber filaments, it is difficult to make the resin 132 enter the inside spaces of the warp 133 and weft 134. The difficulty of impregnation with the resin 132 causes uneveness of the resin in the laminated substrate. This not only exerts a bad influence upon mechanical, thermal, and electrical characteristics but also directly affects the manufacturing speed, thereby causing inefficiency.
In addition, the unsatisfactoriness in impregnating the warp 133 and weft 134 with the resin 132 causes unsatisfactoriness in adhesion of laminated glass fiber fabrics 131, and accordingly, causes delamination.
As one of the other important characteristics required for a substrate for a printed circuit, there is a hole-forming working property. The hole-forming working is an important process for boring holes to fix parts mounted onto the substrate and to connect circuits on the respective layers. The hole-forming working is generally categorized into two methods of punching working and drilling working. Of course the punching working is low in cost. Although the former has been mainly used to paper phenol type substrates, it has been not applied to substrates using glass fiber fabrics because of the severe abrasion in molds, the poor result in finishing on the inner walls of punched holes, etc. Therefore, the latter has been generally used to the substrates of glass fiber fabrics.
However, also in the drilling working, abnormal cracks and breaks have occur during hole cutting on occasion. That is, glass fiber fabric has filaments not opened to make impregnation with resin unsatisfactory, so that there arises unevenness in composition of glass fiber fabric and resin in the laminate. The unevenness is unsuitable for drilling working. Accordingly, the drilling speed cannot be increased during drilling working; cracks and breaks arise easily; and hole-walls cannot be finished up with a good result.
The abnormality of the hole-walls causes incompletion of through-hole portions, resulting in lowering in reliability of circuits. As described above, hole-forming working in printed circuit substrates is an important factor for the reliability of circuits depending on the kind of the printed circuit substrate, and this factor greatly affects workability in manufacturing the substrates.
To improve the characteristics in surface smoothness and impregnation, conventionally, various methods, such as the improvement in form of threads forming fabric, the use of nonwoven fabric, the application of shear stress to fabric by a roller, etc., have been proposed. Further, to improve the characteristics in delamination of glass fiber fabric, various methods, such as a method of gigging fabric by needle punching working, or the like, have been proposed. These methods are effective to improve the surface smoothness and delamination in the case of use of nonwoven fabric. However, the filaments of nonwoven fabric are not mutually intertwined and not restricted, so that the form of nonwoven fabric is kept by an adhesive agent. Therefore, the adhesive agent must match with the resin with which the nonwoven fabric is impregnated. In addition, since the filaments or threads of nonwoven fabric are not mutually restricted, the glass fiber content of nonwoven fabric is a half or less (about 30%) of that of woven fabric. Accordingly, the reinforcement effect by glass fiber is, of course, low, so that nonwoven fabric has a defect of poor dimensional stability, and the above-mentioned problems have been not completely solved by the improvement in threads forming fabric, the application of shear stress by a roller, etc.
Further, in the case where needle punching is performed onto fabric to improve delamination troubles, such as destruction of constituent threads by complete cutting off, displacement of seams, or partly-not-needle-punched fabric, often arises. Accordingly, it is impossible to perform uniform gigging and densely fuzzing working in practice. The cut-off destruction of constituent threads causes unevenness of fabric. In addition, since needle punching working provides punched and non-punched portions so that surface smoothness may be deteriorated.
Further, to improve the hole-forming workability, there have been proposed attempts such as use of glass fiber paper as a basic material or use of glass fiber paper in the core portion of a substrate so that glass fiber fabrics are stacked onto the opposite surfaces of the core glass fiber paper (CEM-3 type). Although the hole-forming workability is improved and punching working is feasible to a certain degree by the attempts, glass fiber content is not improved in the case of using glass fiber paper as well as in the above-described case of using nonwoven fabric. Accordingly, the attempts are unsatisfactory in the reinforcement effect.
Recently, in the field of electronics appliances mount techniques, a surface mount technique (SMT) has become to be widely used. One of the most remarkable characteristics of a printed-circuit substrate for SMT is that the substrate is formed with a lot of via-holes of a small diameter (0.3 mm or less), and therefore it is an important problem how to cut such via-hole with high accuracy, high reliability and high efficiency in a thick substrate, that is, in a substrate having a large aspect ratio.
In the printed-circuit substrate using conventional glass fiber fabrics, however, there have been difficulties in accuracy in positions of and change in dimensions of the via-holes in boring holes by using a drill of a small diameter. Accordingly, it is required to improve the glass fiber per se.
That is, in the case where hole-boring is performed by using the small-diameter drill in a substrate having a large aspect ratio, there is such a defect that the top of the drill strikes against the glass fiber so that the hole formed in the rear surface displaces, as indicated by a solid line in FIG. 6, so as to make it difficult to form a vertical via-hole. Even in the case where via-hole could be formed accurately in a substrate, there is a possibility that a dimensional change arises in the substrate after after-treatments such as etching, heating, etc., so that the accurate via-holes become distorted.